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Monday, December 15, 2014

11 minutes before sunset

The sun set here at 4:13 PM. And tonight, Monday, it will be 4:13 again. And then - yippee! - it starts getting later, with Tuesday sunset at 4:14.

The sun will keep on rising on its old schedule, though, later each day until January 5th, at 8:06 AM, with the next day's sunrise at 8:05. By then, sunset will be falling after 4:30.

But the solstice, with the shortest day of the northern hemisphere's year, falls this year on the 21st. We are scheduled to get 8 hours, 13 minutes, and 42 seconds of daylight today. (Those 42 seconds are precious!) By the 21st, we'll be down to 8 hours, 11 minutes, 57 seconds.

(We're lucky; up in Inuvik, directly north of us, today they'll get a total of zero (0) seconds of daylight. It won't be until January 6th that they'll have a sunrise.)

This is all very confusing. Why are the sunset and sunrise times, and day lengths, on different schedules? I found an explanation on the Royal Museums Greenwich site (UK).

The winter solstice is the time when the Sun reaches its southmost distance from the celestial equator and hence, in northern latitudes is the day when the Sun is lowest in the sky at noon. This is, naturally, the shortest day of the year in northern latitudes. To many people it seems odd, therefore, that the time of sunrise continues to get later in the day after the solstice.

The reason for this is that the Sun does not cross the meridian (when it is highest in the sky) at precisely noon each day. The difference between clock-defined noon and the time when the Sun is on the meridian is called the Equation of Time and represents the correction which must be applied to the time given by a sundial to make it agree with clock time.

There are two reasons why the Sun is not on the meridian at noon each day. The first is that the path of the Earth around the Sun is an ellipse, and not a circle. The second is that the Earth's equatorial plane and its orbital plane are inclined to one another. The two effects add together to yield the equation of time which can amount to some 16 minutes difference between solar and mean time.

Oh, but here's a bit more info to add to the confusion!

The Equation of Time

Very simply, the equation of time is the difference between time that is measured using a sundial (true or apparent solar time) and time that is measured using a watch or a clock (mean solar time).

Most clocks work on the idea that a day - the time between one noon to the next - is exactly 24 hours.

Scientifically, however, a day is defined as the duration between 2 solar noons. A solar noon is the time of the day when the Sun is at the highest point in the sky, and a solar day is the duration between two solar noons.

A solar day is not exactly 24 hours long. Its length varies throughout the year. In fact, the solar day is longer than 24 hours around the summer and winter solstices and is shorter than 24 hours around the spring and fall (autumn) equinox. This means that the length of the solar day does not always match up to the length of a day as measured by a clock.